Pebble heat exchange conversion process



Patented July 14, 1953 PEBBLE HEAT EXCHANGE CONVERSION PROCESS Harold Joseph Hepp, Bartlesville, kla., assignor to Phillips Petroleum Company, a corporation of Delaware Application May 31, 1949, Serial No. 96,297

13 Claims.

This invention relates to a process and apparatus for the thermal conversion of hydrocarbons utilizing pebble heat-exchange. A specific aspect of the invention pertains to the cracking of light hydrocarbons to less saturated hydrocarbons by pebble-heat exchange.

Conventional pebble heat-exchange processes entail gravitating a contiguous compact stream of pebbles thru an upper chamber in contact with hot combustion gas and then gravitating the hot pebbles thru one or more relatively narrow pebble passageways to a lower pebble heat-exchange chamber where the hot pebbles are utilized to heat a suitable feed gas, such as a hydrocarbon, in countercurrent flow. In order to make it feasible to withdraw the reaction product or treated gas from the upper portion of the lower heat-exchange chamber the connecting pebble passageway (or throat) is extended into the lower chamber a short distance so as to provide a gas collecting or vapor space above the pebble bed for collection and withdrawal of the gas thru one or more communicating conduits. It has been found that when efiecting hydrocarbon conversion processes at high temperatures in this type of apparatus it is difficult if not impossible to avoid coke formation and deposition in the vapor space above the pebble bed in the lower chamber. Coke formation is caused by the overcracking of the hydrocarbon being treated due to soaking of the hydrocarbon or products of the conversion in the vapor space, which condition is aggravated by the difiiculty of rapidly withdrawing the hydrocarbon effluent from the upper portion of the reactor. The copending application of M. O. Kilpatrick and H. J. Hepp, Serial No. 699,666, filed September 27, 1946, now Patent No. 2,559,957 is concerned with the avoidance of carbon deposition in the upper portion of a pebble heater reactor and offers one solution to this problem. The invention there is in the design of the reactor which eliminates the projection of the pebble throat into the upper portion of the reactor by disposing the throat at the periphery of the reactor and streamlining the flow of hydrocarbon from the top of the pebble bed thru a gradually converging top closure member into a gas take-off.

The present invention is concerned primarily with the elimination of carbon deposition in the upper portion of the lower pebble heat-exchange chamber of a pebble heat-exchanger apparatus.

The principal object of the invention is to provide a pebble heat-exchange process and apparatus for the. thermal conversion of hydrocarbons which .prevents'coke formation on the walls and throat in the upper portion of the lower pebble heat-exchange chamber of a two-chamber pebble heater apparatus. It is also an object of the invention to provide an economical and thermally efiicient pebble heat-exchange process and apparatus for the conversion of hydrocarbons. Another object is to provide an improved process for cracking light hydrocarbons, including propane, ethane, and methane to less saturated hydrocarbons, such as propylene, ethylene, and acetylene; and for cracking ethylene and/ or propylene with or without admixed acetylene to butadiene, cyclopentadiene, and aromatics. A further object of the invention is to provide an improved process for cracking methane to acetylene and aromatic oils; Other objects of the invention will become apparent from a consideration of the accompanying disclosure.

The invention eliminates the gas collecting or vapor space in the'upper portion of the lower chamber of a two-chamber pebble heater apparatus by sloping the top closure member and refractory lining at an angle with the horizontal greater than the angle of repose of the pebbles used in the process. This sloping top closure member joins the throat or pebble passageway connecting the two heat-exchange chambers in such a manner as to avoid the formation of a vapor space between the pebble bed and the refractory walls of the apparatus, thereby preventing the formation of carbonaceous deposit in this area when the apparatus is being used in the high temperature conversion of hydrocarbons. This structure is made possible by the elimination of the necessity for withdrawal of gas or vapor from the upper portion of the lower heat-exchange chamber. Instead the hydrocarbon being treated is passed upwardly thru the pebble throat which also serves as a combustion chamber to whioha regulated amount of oxygen-containing gas, such as air. or pure oxygen, is introduced so as toburn a suificient portion of the hydrocarbon to supply the heat necessary to effect the desired conversion of the hydrocarbon in the pebble heatexchanger system. Hydrocarbon conversion is effected in the pebble passageway-combustion chamber between the pebble heat-exchange chambers and continues in the lower portion of the upper chamber. Heat-exchange between the hot reaction products and the cooler pebbles takes place in the upper portion of the upper heat-ere changer so as to recoyerheat from the reaction efiluent and render the process more thermally efficient.

Pebbles utilized irrthe-process of the invention are preferably small spheres ranging in diameter from to 1%" compacted from alumina, mullite, alumina-mullite, magnesia, zirconia, thoria, periclase, Monel, inconel, and similar highly refractory materials. The ceramic materials used in manufacturing pebbles must be heat treated at high temperature so as to bond and glaze the pebbles to make them more rugged and attrition resistant in gravitating heat-exchange operation. Non-spherical pebbles are generally economically impractical because of the excessive attrition losses suffered in gravitating bed processes. Pebbles, particularly of alumina and mullite, have been developed which withstand temperatures above 3080 F. and which suffer very little attri tion loss.

For a more complete understanding of the invention, reference is made to the drawing which is a plan view, partly in section, of a preferred modification of the invention.

Pebble heat-exchangers I0 and I I in connecting throat or pebble passageway I2 are vertically aligned to facilitate the gavitation of a compact contiguous stream of pebbles thru the apparatus from pebble inlet conduit I3 to pebble feeder or flow regulator I4 in pebble chute I5 leading to elevator I 6. Pebble feeder I l may be any suitable device, such as a table feedenstar valve, or a vibrating screen type feeder which is regulable so as to control the flow of pebbles thru the unit. Elevator I6 may be of the bucket, screw, or gas lift type. Line I! is a feed line to the lower portion of chamber II and delivers feed gas to a suitable gas distributing means I8. Line I9 is a take-off line for the efiluent from the process, while line 2I serves to introduce an oxygen-containing gas to the interior of throat I2 thru the annular gas space 22 formed by concentric cylindrical refractories 23 and 24. The downwardly and inwardly sloping openings or slots 25 distributed uniformly at short intervals around the refractory wall 24 serve to conduct oxygen-containing gas from annular space 22 to the pebble stream 25 in the interior of throat I2 without admitting pebbles to annular space 22. While lines I'I, I9, and 2| are shown as single lines, a plurality of lines may be utilized to perform their function. Pebble chute I3 extends into the upper portion of chamber Ill a substantial distance so as to form a pebble bed having a top surface indicated at 27, thereby forming a gas collection or vapor space 28 above the pebble bed.

Chamber 3! connected with pebble chutes I3 and I5 by pebble conduits 32 and 33, respectively, is a pebble regeneration chamber which is utilized for burning off the carbon deposited on the pebbles during their passage thru pebble chambers It, II, and I2. Lines 34 and 35 serve to pass oxygen-containing gas to and withdraw combustion gas from chamber 3|. Flow control valves 3% and Si serve to regulate the rate of flow of pebbles thru regeneration chamber 3I. Pebble conduit 38 and valve 39 provide an alternative flow of pebbles from regeneration chamber 3I into the system thru the upper portion of chamber II Lines ll, 22, 43, and 44 serve to introduce steam or other blocking gas to confine the hydrocarbon to the treating zones.

In operation a suitable hydrocarbon feed, usually in gas or vapor form, is introduced into the lower portion of chamber II thru line H where it is distributed by gas distribution means 8 for uniform flow upwardly thru heatexchanger II in countercurrent flow to a gravitating stream of pebbles which have been heated .of the apparatus and process.

by combustion of a portion of the hydrocarbon in pebble passageway-combustion chamber I2 and in the lower portion of the upper heat-exchanger It. The hydrocarbon is preferably heated to approximately the temperature at which conversion, such as cracking, takes place. By the time it reaches the entrance to the combustion chamber, airor other oxygen-containing gas introduced thru line 2i and distributed thru gas space 22 passes thru slots 25 at relatively high velocity into the void spaces of .the gravitating contiguous, compact stream of pebbles 26 in the combustion chamber and burns a suitable portion of the hydrocarbon ascending this passageway. Sufficient hydrocarbon is burned to supply the endothermic heat of reaction required and to make up for the loss of heat thru radiation and in the effluent take-off thru line IS. If the hydrocarbon is not preheated to reaction temperature on its ascent thru pebble-heat exchanger II, sufficient heat must also be provided by combustion to raise the temperature of the hydrocarbon to reaction temperature. The conversion initiated in combustion chamber I2 is continued in the lower portion of conversion heat-exchanger I8 until the reaction is stopped by contact with cool pebbles descending chamber I0.

It is desired to control the process so that the efiiuent passing out thru line I9 is reduced to a temperature below 1000 F., and preferably below 800 F., so as to increase the thermal efficiency Likewise, it is desirable to cool the pebbles descending heatexchanger I I to a temperature below 1000 F. and preferably below 800 F. By maintaining a low pebble exit temperature at the bottom of pebble heat exchanger I I, heat losses in the pebble conduits and elevator between the pebble outlet from chamber II and the pebble inlet to chamber iii are kept to a minimum. This method of operation also permits the use of ordinary cast iron or steel equipment in this part of the apparatus and thereby substantially reduces the cost and maintenance of equipment.

Outlet pebble and gas temperatures may be controlled in part by the depth of the pebble beds in chambers I0 and II. In general, the deeper the pebble bed the more complete the heat exchange and the lower the ultimate temperature differential between pebbles and gas. However, the length of chambers I0 and II should not be so great as to effect an undesirable pressure drop thru the gravitating pebble bed. It is obvious that the maintenance of a low pebble outlet temperature from heat-exchanger II aids in maintaining a low gas temperature in line I9 in any given unit.

In the conversion of hydrocarbons, especially involving cracking, in pebble heat-exchange apparatus such as that shown in the drawing, when operated according to the invention carbonaceous material is deposited on the pebbles in the upper portion of heat-exchanger II, in combustion chamber I2, and in the lower portion of heat-exchanger I6. In order to maintain the pebbles sufiiciently free of carbon for efficient heat-exchange and to avoid agglomeration of the pebbles, the invention provides for the withdrawal of a portion of the pebbles during each cycle thru valve 35 so as to pass the pebbles in a contiguous stream thru line 32, regeneration chamber BI, and line 33 back to pebble chute I5. In regeneration chamber 3| the carbon coated pebbles are contacted with a stream of oxygencontaining gas admitted thru line 34 under suitairspace able conditions for burningofi marathon-ite the ebbles. In this manner, by IQIIIQYiQQtfififiI tain fraction of the pebble stream passing thru conduit l3, removing at. leasta substantial p. tion of the carbon therefrom, and returnin -them to the pebble stream. passingthru. the system, the main pebble stream is. kept .sumciently free of carbon to permit efiicient useof' the pebbles, as heat-exchange material in the process, H the pebbles entering regeneration chamber; arev too cool to support combustion when contacted with air at atmospheric temperature, -the.'.air-.admitted thru line 34 may be preheated torequired temperature. Onthe. other hand; in some cases it might be necessary .to dilute the, oxygenrqcone taining gas so as to maintain sufiicientlyrlow cry..- g-en content to avoid overheating. .Ofr-thfi pebbles. In some cases the pebblespassingeout the :bottom of chamber 3! are too hot to. passcdirectlyinto pebble chute l5 thru line -3.3.andvalve-31 and-it is advantageous to pass them mtdtheupperportion of chamber ll thru line. 3.0ands-yalve '39 so as to utilize their heat content in'chamber. Ii 1* to preheat the hydrocarbon feed .0001 them to suitable transfer temperature.

In one modification of the invention. .irr order V to reduce dilution ofthe. product stream. with combustion gas and to improve the thermal efliciency of the process, valves 45 and: 4-6 areop'erated so that the air .for combustion in combustion chamber I2 is passed thru line 30 and heatexchanger 20 in indirect heat-exchange. the hot eflluent in line l9. This expedient further increases the thermal efficiency of the process by reducing the heat loss in the effluent in line 19-. In some cases it may be desirable to-use a water quench in line l9. 0

The process may be operated-at anysuitable pressure, but pressures just-above atmospheric pressure are preferred. Lines M, 42, 43, and 44 are utilized for admitting steam or other blocking gas to the pebble conduits to confine the gasses in main unit and in regeneration chain bertl.

When converting propane and/or other lower boiling hydrocarbon, it may be desirable in some instances to admit hot combustion gas thru line 2| in order to conserve the more valuable hydrocarbon being converted. Excess airor oitygen may be used, thereby furnishing a further pore tion of reaction heat by oxidation: of the Iced stock by the residual oxygen. This method of operation permits the use of less expensivev fuel gas for furnishing heat or reaction and othdr heat requirements oithe process,

The following example illustrates the inven tion in the cracking of-ethane to produceethylene.

x mple.

bed depth of '7'. Th? upper prtjon orthischaniher is sloped or shaped so as. to avoid anyv open space between the walls and the pebbles.- The outgoing pebbles from preheater H are cooled to about 700 F. While the ethane ispreheated dun-.-

ine its ascentth u the chamb r to about .1199? The preheated eth ne.

' nd-i ur her cod in eatx ng r t b lqr 5 F- On1v a n i e amo nt of rbqn s'den ite on. h up e walls f ch mb r Th efilue i has the fol owin com et tors 5 s r d filue tmer be Worked up n a y desired manner for recovering valuable constituents by methods .lgnown in the art and the ethylene recovered by a low temperature trace tionation. Ethane is desirably recycled to the process.

As a fu th mod fica n o he in nt n, cry

gen may be. introduced thru line 21 in place. 015 air-end wi l m a the ni g n. ue t. i h a t o pro uc thu ed ng il tion and making it easier to recover ethylene and also hy drqeien- 1 T e exam l me il us rates ne t pe. of process for which the invention is particularly suited and is not to be interpreted as unduly and unnecessarily limiting the invention. The invention is applicable to the endothermic conversion .ojshydrocarbons broadly. It has particular utility in the crackin o low m l ula w i ht paraffins and olefins, either singly or in admixture, so as m produce olefins, diolefins, acetylene, aromatics, etc. e

11-. A process for endothermic conversion of a hydrocarbon, comprising gravitating a contiguous compact stream of hot pebbles thru a series of contiguous zones including an upper conversion zone, a lower feed preheating zone, and an interconnecting combustion zone; contacting that section of said pebble stream in said preheating zone with an ascending stream of convertible hydrocarbon so as to preheat same; passing the entire preheated hydrocarbon stream directly thru said interconnecting combustion zone in contact with said pebble stream; burning a sufficient portion of said preheated hydrocarbon stream by introducing oxygen-containing gas into said interconnecting combustion zone into admixture with the hydrocarbon therein to furtherheat and to efiect desired conversion of said hydrocarbon with concomitant heating of the pebbles in said interconnecting combustion zone and in the. lower portion of said conversion zone; and passing the entire sflleent. fr m ai interconnect n combustion zone into said conversion zone and continuing-the '2 conversion in the lower portion of said conversion zone; transferring pebbles from the lower portion of said preheating zone to the" upper portion of said conversion zone; and recovering a converted hydrocarbon effluent from said conversionzone; 2. A process for endothermic conversion of a hydrocarboncomprising gravitating a contiguous compact streamof hot pebbles thru a series of contiguous zonesincluding an upper'conversion zone, a lower feed preheating zone, and an interconnecting combustion zone; contacting that section ofsaid pebble stream in said preheating-zone with an ascending stream of convertible hydrocarbon so as'to preheat" same; -maintaining the upper portionof said preheating zone full' of pebbles so as to avoid stagnant gas spaces with concomitant deposition of carbon therein; passing the entire preheated hydrocarbon stream directly thru said interconnecting combustion zone in contact with said pebble stream; burning a sufficient portion of said preheated hydrocarbon stream by introducing an oxygen-containing gas intosaid interconnecting combustion zone into admixture with the hydrocarbon therein to further heat and to efiect desired conversion of said hydrocarbon with concomitant heating of the pebbles in said interconnecting combustion zone and in the lower portion of said conversion zone; and passing the entire efiluent from said interconnecting combustion zone into said conversion zone and continuing the conversionin the lower portion .of'said so a'sjto preheat same;"maintaining the upper portion of'said preheating zone full of pebbles'so as toavoid stagnant gas spaces with concomitant deposition of carbon therein; passing the preheated hydrocarbon thru said interconnecting zone in'contact with said pebble stream; burning O a'sufiicient portion of said preheated'hydrocarbon stream by introducing an oxygen-containing gas into said interconnecting zone into admixture with the hydrocarbon therein to further heat and to. effect desired conversion of said. hydrocarbon with concomitant heating of the-pebb1es-:in said interconnecting zone and in the lower portion of said conversion zone; continuing the conversion in the lower portion of said conversion'zone whereby carbonaceous material. is deposited on the pebbles; t ransferring pebbles from the. lower portion of said preheating zone to the upper portion of said conversion zone; l passing a portion of the pebble stream after withdrawal from the lower portion of the preheating zone andbefore introduction to the conversion zone thru a' regeneration zone and-there burning 01f at least a portion of the carbonaceous-material from said pebbles withan oxygen-containing gas and re,- turning the regenerated pebbles to said gravitating stream; and recovering a converted hydrocarbon efiluent from said conversion zone.

4. The process ofclaim 3 in which the regenerated pebbles are returned to the conversion 5. The process of claim 3 in which said regenerated pebbles are returned to the preheating zone.

6. The process of claim 2 in which the conversion comprises cracking.

7. The'process of claim 2 in which the conversion comprises cracking a light hydrocarbon.

8. The process of claim 2 in which the conversion comprises cracking methane.

9. The processof claim 2 in which the conversion comprises cracking ethane.

. -10.'-The process of claim 2 in which the conversion comprises cracking propane.

' 11; A process for 'crackinga light hydrocarbon, comprising gra vitating a contiguous compact stream Tofhot pebbles through a series of contiguous Zones including an upper cracking zone, alower'feed preheating zone, and an interconnecting combustion zone; passing a light hydrocarbon feed upwardly through said preheating zone in counterourrent contact with said pebbles so as to heat said feed to a temperature in the rangeof 1000 F. to the-incipient cracking temperature of said feed and cool said pebbles to a temperature below 1000 F.; maintaining the upperportion. of said'preheating zone full of pebblessoas to avoid stagnant gas spaces with concomitant deposition of carbon therein; passing the entire heated *feed upwardly through said combustion zone and there burning such a portion of. samejby admixture with an oxygen-containing-gasto-heat and crack said feed with concomitant heating of the pebbles in the combustion-zone; passing the entire resulting gaseous mixtureupwardly through said cracking zone with continued cracking of the hydrocarbon and heating of pebbles in the lower portion thereof and with cooling of the cracked efiluent and heating of the pebbles in the upper portion thereof transferring pebbles from the lower portion of said preheating zone to the upper portion of said cracking zone; and recovering craokedproduct from the upper portion of said cracking zone.

l2; Theprocess of claim 11 in which carbon- .aceous material is deposited on said pebbles from thecrafeking step and a portion of the pebble stream is continuously passed through a separate an interconnecting combustion zone; passing an ethane-rich feed upwardly through said preheating zone in countercurrent contact with said pebbles so :as .toheat .said feed to a temperature in the rangeof 1400 to 1800 F. and cool said pebbles to a temperature below 1000 F; maintaining the upper portion of said preheating zone full of pebbles so as to avoid stagnant gas spaces with concomitant deposition of carbon therein; passing the entire preheated feed upwardly through said combustion zone and there burning'such a portion of same by admixture Witha preheated oxygen-containing gas as to heat and crack saidfeed to ethylene with concomitant heating of'the pebbles in the'combustion'zo'ne; passing the entire resulting gaseous mixtureupwardly through said cracking zone with continued cracking of ethane in the lower References Cited in the file of this patent Q portion and with cooling of the cracking effiuent UNITED STATES PATENTS and heatmg of the pebbles 1n the upper portion thereof; transferring pebbles from the lower g gg ilia? o t'on of d reheat'n ne to the u e r 1 p r 1 Sal p 1 gm pp rpor 2,486,627 Arnold Nov. 1, 1949 tion of said cracking zone; and recovering an ethylene-rich stream from the upper portion of said cracking zone.

HAROLD JOSEPH HEPP. 

1. A PROCESS FOR ENDOTHERMIC CONVERSION OF A HYDROCARBON, COMPRISING GRAVITATING A CONTIGUOUS COMPACT STREAM OF HOT PEDDLES THRU A SERIES OF CONTIGUOUS ZONES INCLUDING AN UPPER CONVERSION ZONE, A LOWER FEED PREHEATING ZONE, AND AN INTERCONNECTING COMBUSTION ZONE; CONTACTING THAT SECTION OF SAID PEDDLE STREAM IN SAID PREHEATING ZONE WITH AN ASCENDING STREAM OF CONVERTIBLE HYDROCARBON SO AS TO PREHEAT SAME; PASSING THE ENTIRE PREHEATED HYDROCARBON STREAM DIRECLTY THRU SAID INTERCONNECTING COMBUSTION ZONE IN CONTACT WITH SAID PEBBLE STREAM; BURNING A SUFFICIENT PORTION OF SAID PREHEATED HYDROCARBON STREAM BY INTRODUCING OXYGEN-CONTAINING GAS INTO SAID INTERCONNECTING COMBUSTION ZONE INTO ADMIXTURE WITH THE HYDROCARBON THEREIN TO FURTHER HEAT AND TO EFFECT DESIRED CONVERSION OF SAID HYDROCARBON WITH CONCOMITANT HEATING OF THE PEDDLES IN SAID INTERCONNECTING COMBUSTION ZONE; AND PASSING THE ENTIRE OF SAID CONVERSION ZONE; AND PASSING THE ENTIRE EFFLUENT FROM SAID INTERCONNECTING COMBUSTION ZONE INTO SAID CONVERSION ZONE AND CONTINUING THE CONVERSION IN THE LOWER PORTION OF SAID CONVERSION ZONE; TRANSFERRING PEDDLES FROM THE LOWER PORTION OF SAID PREHEATING ZONE TO THE UPPER PORTION OF SAID CONVERSION ZONE; AND RECOVERING A CONVERTED HYDROCARBON EFFLUENT FROM SAID CONVERSION ZONE. 